1. Field of the Invention
This invention relates to maintenance-free sealed lead-acid batteries and more particularly to a maintenance-free type sealed lead-acid battery which is high in the gas absorbing rate, long in the life, low in the cost and high in the vibration-proofness.
2. Description of the Prior Art
There is already suggested a so-called maintenance-free sealed lead-acid battery in which water need not be added, the self-discharge is little and the electrolyte does not leak out. That is to say, in the battery of this kind, in order to prevent the water in the electrolyte from being decreased mostly by the electrolysis of water in the charging end period it is general to utilize a so-called O.sub.2 cycle of re-combining generated oxygen gas by means of negative plates. Also, in order to reduce the self-discharge, a lead alloy containing no antimony or pure lead is used for the grid material. In order to eliminate the leakage of the electrolyte, the electrolyte is made nonfluid. In such sealed lead-acid battery, in order to elevate the efficiency of re-combining oxygen gas by means of negative plates (in other words, the gas absorbing rate) and the leak-proofness, the free electrolyte in the battery must be as little as possible.
There are already suggested two methods of making the electrolyte nonfluid and eliminating the free electrolyte in the battery. One of them is a method wherein the electrolyte is made colloid and is fixed. The other is a system wherein a porous member is impregnated with the electrolyte to make the electrolyte substantially nonfluid.
In using the colloidal electrolyte, generally the structure is simple but there is a defect that, as the internal resistance of the battery increases, the battery performance or particularly the discharge voltage characteristic will reduce.
On the other hand, in such system of impregnating a porous member with the electrolyte as is mentioned in U.S. Pat. No. 3,862,861, generally the characteristic as of the battery is favorable but the price is high. Such porous member as for example, a sheet-shaped separator formed by entangling glass fibers of a diameter not larger than 1 micron as a base with glass fibers of a diameter not smaller than 1 micron or particularly not smaller than 5 microns at random is so high in the active substance retaining function, electrolyte absorbing capacity and gas absorbing rate in the sealed lead-acid battery as to be optimum to the use. However, such glass fibers are so special as to be very expensive and are so low in the physical stiffness that a manufacturing process different from that of the conventional lead-acid battery must be contrived. Also the fiber diameter is so small that the elasticity, active substance pressing ability and vibration-proofness are low. Further, a battery having a separator of three layers of which the outer layers are of glass paper formed of glass fibers of a diameter not larger than 1 micron and the intermediate layer is of latex-bond diatomaceous earth is shown in U.S. Pat. No. 3,753,784. According to the researches made by the present inventors, it is found that, if the structure is thus made of three layers holding the layer low in the electrolyte absorbing force (here, the latex-bond diatomaceous earth layer) with the layers of glass paper in the form of a sandwich and the glass paper layers are made to contact both positive and negative plates, the difference in the electrolyte absorbing force between the outer layer and intermediate layer will be so large that the electrolyte will dry out in the intermediate layer. When the separator is thus formed of three or more layers, the manufacture of the battery will be complicated and, as a result, the obtained battery will be expensive.